A multi-level controller may be used for controlling a multi-phase electrical machine. The multi-level converter includes a phase module for each phase generating a number of different output voltages dependent on the design of the phase module. For example, a two-level phase module generates two output voltages (+UDC, 0) and a three-level phase module generates three output voltages (+UDC, 0, −UDC). A phase module may include a plurality of electrical switches, such as power semiconductor switches, which generate the output voltage of the respective phase according to a switching pattern or switching state, which describes which switches of the phase module are conducting (on) and which switches are blocking (off).
There are several possibilities (e.g., modulation methods) for generating these switching patterns.
For example, switching patterns may be determined with the concept of optimized pulse patterns (OPP). With optimized pulse patterns, a motor's operation may be based on pre-calculated switching patterns that achieve a certain minimization objective, such as the elimination of certain harmonics or the minimization of the total harmonic distortion of the motor current. However, when the motor speed or the amplitude of the voltage or both go below a certain threshold value, the number of pulses required for an optimized pulse pattern is so high that it may become prohibitive. Moreover, at such low values of the motor speed and/or voltage, the usage of optimized pulse patterns does not provide an advantage in terms of the produced value of total harmonic distortion of the motor current, when compared to other methods, such as pulse width modulation (PWM).
Thus, in the case of low motor speed and/or voltage, the concept of pulse width modulation (PWM) may be used. Here, for example, the average value of the output voltage over a modulation cycle that has to be fed to the electrical machine may be controlled by switching between the possible output voltages with a high frequency compared to the fundamental frequency of the AC output voltage.
Another possibility is to use the concept of direct torque control (DTC), in which states of the motor, for example, the torque and the magnetic flux, are estimated and are controlled to stay within their hysteresis bands by switching when the respective variable error reaches its upper or lower limit.
When any modulation method is used for the operation of a converter, in particular for one with a five-level topology, a key challenge arises: the proper choice of the actual converter switching patterns that reproduce the required output voltages while balancing the internal voltages of the converter (for example neutral point potential, floating capacitor voltages).